1. Field of the Invention
This invention relates to a method and an apparatus for growing ZnO single crystal boules. More particularly, ZnO single crystals are grown at a high growth rate by physical vapor transport at an elevated temperature, nominally in the range of 1300-1800° C.
2. Description of the Related Art
Zinc oxide (ZnO) has a wide band gap, high stability and high thermal operating range that makes it a suitable material as a semiconductor for fabricating light emitting diodes (LEDs) and laser diodes, photodiodes, power diodes and other semiconductor devices. ZnO single crystal is also a suitable substrate material for epitaxial growth and fabrication of gallium nitride (GaN) and III-V nitride devices, including light emitting diodes (LEDs), laser diodes, high frequency, high power and high temperature devices. In order to manufacture these semiconductor devices, the ZnO is provided as large single crystals that are used to make ZnO wafers. The wafers are then polished into substrates suitable for epitaxial growth and device fabrication. The quality of the semiconductor is highly dependent on the purity and structural characteristics of the ZnO single crystals.
ZnO bulk single crystals have been grown mainly using three known techniques: (a) chemical-assisted vapor transport (CVT), (b) hydrothermal growth, and (c) high-pressure melt growth. ZnO crystals up to 2 inches in diameter have been manufactured using each of these techniques. However, each of the three growth techniques has drawbacks that severely limit its capability to produce high quality ZnO crystals in an efficient manner at low cost.
In CVT technique, a source material and a seed crystal are placed inside a growth furnace in such way that the temperature of the source material is higher than that of the seed and a chemical agent is used as both a sublimation activator and a transport agent to carry vapor species and deposit them onto the seed to form single crystals. CVT technique is useful for growing crystals at low temperatures where the vapor pressure due to sublimation of the materials is too low to allow a growth using a sublimation-and-re-condensation process. Because the vapor pressures of ZnO at temperatures between 900-1200° C. are low, CVT ZnO growth technique uses hydrogen gas (or a mixture of hydrogen and water vapor) as a vapor transport agent so that an appreciable growth rate can be achieved. CVT ZnO is usually carried out in a sealed or a slightly “leaky” quartz crucible. When a “leaky” crucible is used, the ZnO single crystal can be grown without touching the crucible wall, which minimizes the stresses in the crystal. The maximum growth rate in CVT ZnO growth technique using hydrogen gas is about 0.1 mm/hr.
Hydrothermal growth technique is conducted in an aqueous solution inside a high-pressure autoclave at a temperature between 300-400° C. Growth rate of ZnO in hydrothermal technique is about 0.04 mm/hr (1 mm/day). The inherent low growth rates in CVT and hydrothermal techniques for ZnO single crystals severely limit their use in large volume production of ZnO crystal wafers, despite the relative good quality of the ZnO crystals produced by these two techniques.
The third technique for ZnO crystal growth is a high-pressure melt growth technique developed by Cermet, Inc of Atlanta, Ga. In the high-pressure melt growth technique, a ZnO crystal boule is grown from a ZnO melt contained in a water-cooled crucible under an oxygen pressure up to 100 atmospheres. The high oxygen gas pressure is required to suppress evaporation and dissociation of ZnO melt at high temperatures. Although this technique can potentially grow ZnO crystals at a high growth rate (>1 mm/hr), the inherently high dissociation pressure and high evaporation rate of ZnO melt make it difficult to control the crystal growth process and the crystal quality. Moreover, the cost of purchase and operation of the high-pressure melt-growth furnaces for ZnO crystals is expected to be extremely high because of the extremely high gas pressure requirements combined with the high temperature requirements.
In addition to the three above techniques, another growth technique is physical vapor transport (PVT) that is widely used for growing crystals whose vapor pressure due to sublimation is sufficiently high at high temperatures. PVT is essentially a sublimation and re-condensation process, in which a source material and a seed crystal are placed inside a growth furnace in such way that the temperature of the source material is higher than that of the seed so that the source material sublimes and the vapor species diffuse and deposit onto the seed to form single crystals. PVT growth technique has been used for growing many semiconductor crystals, such as zinc selenide (ZnSe) and silicon carbide (SiC) crystals. In both PVT and CVT growth techniques, crystals may also be grown through spontaneous nucleation without using a crystal seed.
PVT growth processes for ZnSe single crystals in the temperature range of 1000-1250° C. have been demonstrated. Similar to CVT growth of ZnO, PVT growth ZnSe with crystal boules not touching the crucibles were also demonstrated in “leaky” crucibles. But the growth rate demonstrated to date in PVT ZnSe growths is less than 0.1 mm/hr, which is too low to be adopted for volume production of ZnSe wafers for semiconductor device applications. The low growth rate in the PVT ZnSe growths is due to the low vapor pressures of ZnSe at the relatively low growth temperatures (1000-1250° C.).
There remains a need for a process and apparatus to rapidly grown ZnO single crystals that do not have the disadvantages of the prior art as described above.